Alzheimer's disease (AD) is a fatal degenerative disorder of the central nervous system that affects an estimated 3%-4% of the United States population above the age of 65 (Katzman, Arch. Neurol. 33:217-8 (1976)). AD is characterized by profound memory impairment, emotional disturbance, and in late stages, personality changes (Bartolucci et al., Proteins 42:182-91 (2001)). Molecular symptoms include neuronal loss, synaptic damage, and increased levels of neurofibrillary tangles, neuritic plaques, and granulovacuolar degeneration. The reduced cognitive function seen in patients with AD are thought to be primarily related to the degeneration of cholinergic neurons in the cortex and hippocampus, which results in deficits of cholinergic transmission and reduced levels of acetylcholine (Scott et al., Drugs 60 (5):1095-1122 (2000)). Studies have shown that AD is associated with decreased levels of choline acetyltransferase (CHAT) and nicotinic acetylcholine receptors (nAChRs) (Bartolucci et al., supra).
Since there is no cure for AD at the present time, current treatment for AD patients focuses on relieving some of the symptoms associated with this disease. The major strategies revolve around increasing central cholinergic function by elevating the transient levels of acetylcholine in the brain (cholinergic therapy). Current drugs for elevating acetylcholine levels are AChE inhibitors, which decrease the degradation of acetylcholine in the synaptic cleft, allowing for increased neuronal transmission, and nicotinic agonists, which directly enhance the function of nAChRs (Scott et al., supra; Bartolucci et al., supra).
Cholinergic therapy may also have beneficial effects for mild or minimal cognitive impairment (MCI). MCI is a condition characterized by subtle cognitive deficits not severe enough to be classified as true dementia, but in many patients represents an early stage of AD (Almkvist et al., J. Neural Transm. Suppl. 54: 21-29 (1998)). Thus, if drug therapy to enhance cognition is started when the symptoms of dementia first appear, even before a clinical diagnosis of AD, it is possible that the onset of AD may be delayed (Small, Hippocrates 14(9) (2000)). Other cognitive disorders that may benefit from cholinergic therapy are vascular dementias and Lewy body dementias.
One compound that has been approved in the United States for the treatment of mild to moderate dementia of the Alzheimer's type is galantamine, which is a tertiary alkaloid, and marketed as Reminyl® (galantamine hydrobromide) by Janssen Pharmaceuticals (Scott et al., Drugs 60(5):1095-1122 (2000)). Although clinical trials have established galantamine's efficacy in producing significant improvement in cognitive function and activities of daily living in AD patients as compared to placebo treatment (Raskind et al., Neurology 54:2261-8 (2000); Coyle et al., Biol. Psychiatry 49:289-99 (2001); Rockwood et al., J. Neurol. Nerurosurg. Psychiatry 71:589-595 (2001)), it is not clear whether or not all patients with mild to moderate AD will ultimately demonstrate a clinically meaningful improvement in cognitive function, suggesting that there may be variability in an individual's response to pharmaceutical agents to treat cognitive impairment. However, physicians currently are unable to identify patients who are at risk for reduced or lack of efficacy of galantamine therapy, which can be expensive and is not without risk of side effects, with the most common side effects being nausea, vomiting, diarrhea, dizziness and anorexia (Wilcock et al., BMJ 321:1-7 (2000); Scott et al., supra). Thus it would be useful to understand the biological basis for the variability of response to galantamine.
Variability in the efficacy and toxicity of a number of drugs has been correlated with genetic variation in proteins involved in drug metabolism (Evans et al., Science 286:487-91 (1999)). Metabolism of galantamine is primarily mediated by the cytochrome p-450 enzyme system, specifically the isozymes 2D6 and 3A4 (CYP2D6 and CYP3A4) (Scott et al., supra). Poor CYP2D6 metabolizers exhibit about 25% less clearance of galantamine than extensive CYP2D6 metabolizers, although this difference is not considered to be clinically relevant because the recommended dosage regimen is to individually titrate the dose to tolerability (Reminyl® tablets prescribing information, Janssen Pharmaceutica Products, March 2001). In addition, while several metabolites of galantamine inhibit AChE in vitro, their in vivo activity is not considered to be clinically relevant (Scott et al., supra).
Another potential source of variability of response to galantamine could be genetic variation in proteins involved in the etiology of AD or its severity, or in the mechanism of action of galantamine. For example, as many as 70% of AD patients have a particular single nucleotide polymorphism in the gene encoding apolipoprotein E (the ApoE4 allele) that appears to be correlated with a greater impairment of cholinergic function and a study with the ACHE inhibitor tacrine suggested that the presence of this polymorphism is correlated with reduced response to treatment (Farlow et al., Neurology 50:669-77 (1998)). However, other studies with galantamine showed no significant difference in efficacy of response between patients with zero, one or two copies of the ApoE4 allele (Raskind et al., supra; Aerssens et al., poster presented at 7th International World Alzheimer's Congress, Jul. 9-18, 2000, Washington, D.C.).
One protein whose deregulation may be involved in neurodegenerative disease is CDK5, a serine/threonine kinase (Zheng et al., Eur. J. Biochem. 269(18):4427-34 (2002)), which is encoded by a gene on chromosome 7q36 that consists of 12 exons. Required for normal mammalian central nervous system development, CDK5 is activated by its neuron-specific activator, p35, and phosphorylates both high molecular weight neurofilaments and microtubule-associated protein tau (Hashiguchi et al., J. Biol. Chem. 277(46):4425-30 (2002)). p25, a calpain digested tuncated form of p35, has been found to accumulate in the neurons of Alzheimer's patients (Tseng et al., FEBS Lett. 523(1-3):58-62 (2002)). Unlike p35, p25 is not readily degraded, and the binding of p25 to CDK5 constitutively activates CDK5, changes its cellular location, alters its substrate specificity, and increases its phosphorylation of serine(202)/theonine(205) in tau (Hashiguchi et al., supra). This hyperphosphorylation reduces the ability of tau to associate with microtubules leading to cytoskeletal disruption, morphologic degeneration, and apoptosis (Patrick et al., Nature 402(6762):615-22 (1999); Liu et al., FEBS Lett. 530(1-3):209-14 (2002)).
Recently, various haplotypes of the CHRNA2, EPHX2, and LRPAP1 genes—three genes either directly or indirectly involved in the etiology of cognitive disorders, namely AD in the case of CHRNA2 and LRPAP1, and Parkinson's Disease in the case of EPHX2—were discovered to exist in a cohort of Alzheimer's patients, and were discovered to be associated with a response to galantamine therapy. Because of its involvement in the etiology of AD, CDK5 is a logical candidate for a determination of whether CDK5 haplotypes are similarly associated with response to galantamine therapy.